Add documentation for Apple Silicon Installation

[picturedots]

The documentation for Apple Silicon installation is missing. I believe the instructions that should be added are

export PROVIDER=all
curl -LO "https://github.com/GoogleCloudPlatform/terraformer/releases/download/$(curl -s https://api.github.com/repos/GoogleCloudPlatform/terraformer/releases/latest | grep tag_name | cut -d '"' -f 4)/terraformer-${PROVIDER}-darwin-arm64"
chmod +x terraformer-${PROVIDER}-darwin-arm64
sudo mv terraformer-${PROVIDER}-darwin-arm64 /usr/local/bin/terraformer

This should resolve #590

NOTE: PROVIDER should be set to "all" because, as per the releases page the only two options for darwin-arm are "all" and "azure".

[github-actions]

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